There are many signal transduction systems in cells which are functionally linked to each other to control the proliferation, growth, metastasis and apoptosis of cells (Kaelin, Nature Reviews Cancer, 2005, 5:689). The breakdown of the intracellular controlling system by genetic and environmental factors causes abnormal amplification or destruction of the signal transduction system leading to tumor cell generation (Hanahan and Weinberg, Cell, 2000, 100:57).
Protein tyrosine kinases play important roles in such cellular regulation (Melnikova and Golden, Nature Reviews Drug Discovery, 2004, 3:993), and their abnormal expression or mutation has been observed in cancer cells.
The protein tyrosine kinase is an enzyme which catalyzes the transportation of phosphate groups from ATP to tyrosines located on protein substrates. Many growth factor receptor proteins function as tyrosine kinases to transport cellular signals. The interaction between growth factors and their receptors normally controls the cellular growth, but abnormal signal transduction caused by the mutation or overexpression of any of the receptors often induces tumor cells and cancers.
Protein tyrosine kinases have been classified into many families in accordance with their growth factor types, and epithelial cell growth factor (EGF)-related EGF receptor (EGFR) tyrosine kinases, in particular, have been intensely studied (Hynes and Lane, Nature Reviews Cancer, 2005, 5:341). An EGFR tyrosine kinase is composed of a receptor and a tyrosine kinase, and delivers extracellular signals to cell nucleus through the cellular membrane. Various EGFR tyrosine kinases are classified based on their structural differences into EGFR (Erb-B1), Erb-B2, Erb-B3 and Erb-B4, and each of the above members can form a homodimer- or heterodimer-signal delivery complex. Also, the overexpression of more than one of such heterodimers is often observed in malignant cells. In addition, it is known that both EGFR and Erb-B2 significantly contribute to the formation of heterodimer-signal delivery complexes.
Several drugs as small molecules for the inhibition of EGFR tyrosine kinases have been developed, e.g., Gefitinib, Erlotinib, Lapatinib, and others. Gefitinib or Erlotinib selectively and reversibly inhibits EGFR, and Lapatinib simultaneously inhibits EGFR and Erb-B2, thereby arresting the growth of tumors to significantly extend the life time of the patient and provide therapeutic advantages.
Fibroblast growth factor receptor (FGFR) tyrosine kinases consisting of about 800 amino acids are one of Class V receptor tyrosine kinases having three immunoglobulin (Ig)-like domains, i.e., D1, D2 and D3. FGFR tyrosine kinases are classified into FGFR1, FGFR2, FGFR3 and FGFR4, and specifically classified into 48 families. A soluble FGFR tyrosine kinase (FGFR5) has 4 families. A fibroblast growth factor (FGF), a FGFR ligand, is a heparin-binding growth factor, and 23 types thereof have been reported. Generally, one of FGF tends to activate several FGFR tyrosine kinases, however, FGF-7 induces to activate only FGFR2B.
A non-covalent homodimer or heterodimer complex of FGFR formed by binding FGF with immunoglobulin-like domains II and III induces autophosphorylation in an activation loop of an FGFR kinase domain. The autophosphorylation site of FGFR1 binds to an Src homology 2 (SH2) domain of phopholipase C (PLC), and the interaction thereof induces phosphorylation and activation of PLC (Hubbard, Progress in Biophysics & Molecular Biology, 1999, 71:343), which successively initiates a signal transduction through MAPK or PI3K/AKT cellular pathway.
The signal transduction induced by FGF/FGFR is associated with the cell differentiation, proliferation, apoptosis and vascularization, especially it has been known to play an important role in fetal generation and wound treatment. However, abnormal signal transduction caused by the FGF/FGFR overexpression and activated mutation often leads to tumor cell generation, e.g., bladder cancer, breast cancer, prostatic carcinoma, gastric cancer, lung cancer, blood cancer, and the like.
For example, an activated FGFR3 mutation and loss of heterozygosity (LOH) of chromosome 9 are most related to generation of superficial urothelial cell carcinoma (UCC). Also, it has been well known that the activated FGFR3 mutation such as S249C point mutation is strongly related to generation of non-infilltrative bladder cancer (Sibley et al., Oncogene, 2001, 20:4416).
The bladder cancer induced by an FGFR3-S249C mutant can be treated by inhibiting cell proliferation using FGFR3 shRNA or RNAi. The FGFR3-S249C mutant forms a disulfide bond with a molecule, thereby initiating a heterodimer formation of an FGFR extracellular domain, which makes FGFR to maintain its activated state (Tomlinson et al., Oncogene, 2007, 26:5889). Also, it has been recently reported that H-Ras mutation is found in about 30% of the patients suffering from the urothelial cell carcinoma (Dinney et al., Cancer Cell, 2004, 6:111).
FGFR3 has been known to generate hematological malignancies such as multiple myeloma (MM), and the multiple myeloma is caused by dysfunction of t (4; 14)(p16.3; q32.3) chromosome containing FGFR3 in about 15% of MM patients (Chesi et al., Nat. Genet., 1997, 16:260). Moreover, K650E point mutation induced by a modified FGFR3 gene has been reported to generate thanatophoric dysplasia type II (Tavormina et al., Hum. Mol. Genet., 1995, 4:2175). CHIR-258 (TKI258), a benzimidazoloquinolinone derivative, inhibits various tyrosine kinases, especially strongly inhibits FGFR3, and is currently in the clinical stage.
G374R mutation in the activation loop of an FGFR3 kinase domain is found in about 98% of patients suffering from achondroplasia (Richette et al., Joint Bone Spine, 2008, 75:125). The activated FGFR3 induced by G374R mutation in chondrocytes leads to a premature synchondrosis closure and facilitates osteoblast differenciation. Also, the signal transduction of the activated FGFR3 induces to increase bone morphorgenetic protein 7 (BMP7), or to inhibit an expression of Noggin (BMP antagonist) mRNA with a MAPK-dependent manner (Matsushita et al., Human Molecular Genetics, 2009, 18:227).
FGFR3b and FGFR3c mutants were found in 93% of patients suffering from cervix carcinomas, and the activated FGFR3 mutants (e.g., S249C, G372C and K652E) were found in 25% of the cervix carcinomas (Cappellen et al., Nat. Genet., 199, 23:18).
Meanwhile, FGFR3 mutants were found in about 40% of patients suffering from seborrheic keratose. Among of which, most point mutations were R248C, and A393E mutation was also observed in a relatively low frequency (Hafner et al., J. Invest. Dermatol., 2006, 126:2404).
Vascular endothelial growth factor receptor-2 (VEGFR-2) has been known as a kinase insert domain-containing receptor/fetal liver kinase (KDR/Flk-1), belongs to Class III in a subclass of the receptor tyrosine kinases, and closely associates with angiogenesis. Angiogenesis may generate cancer, rheumatic diseases, diabetic retinopathy and neovascular glaucoma. VEGFR-2 is considered as an important molecular target for anticancer treatments based on the fact that inhibition on VEGFR-2 can result in inhibition of angiogenesis. In this connection, various VEGFR-2 low-molecular inhibitors have been discovered and most of them are currently in the clinical stage (Schenone et al., Curr. Med. Chem., 2007, 14:2495). For examples, Sorafenib and Sunitinib are commercially marketable against various tyrosine kinases including VEGFR-2.
Tie-2, which is another receptor tyrosine kinase associated with angiogenesis, is extensively expressed in vascular endothelial cells, and also found in haematopoietic cells. Angiopoietin known as a ligand of Tie-2 is divided into Ang 1 and Ang 2, Ang 1 causing autophosphorylation of Tie-2 by binding to the extracellular domain thereof, and Ang 2 playing an important role in a lymphatic vascular system (Davis et al., Cell, 1996, 87:1161). In an experiment using a mouse, it has been confirmed that angiogenesis and growth of tumor cell became blocked by inhibiting Tie-2 (Lin et al., Proc. Natl. Acad. Sci., USA 1998, 95:8829).
Rearranged during transfection (RET) is one of receptor tyrosine kinases expressed in protooncogens of nerve and excretion systems. An N-terminal extracellular domain of RET is composed of cadherin-like repeats (CLR), a calcium-binding site, nine N-glycosylation sites, and a cysteine-rich region (Airaksinen et al., Nat. Rev. Neurosci., 2002, 3:383). The N-terminal extracellular domain of RET comprises at least 12 tyrosines having an autophosphorylation capability. For example, there are 16 tyrosines in the extracellular domain of RET9. A GFL/GFR complex causes the autophosphorylation and activation of the kinase domain by binding to the extracellular domain of RET (Aiaksinen et al., Nat. Rev. Neurosci., 2002, 3:383). RET has been reported to play an important role in development and increasing of parasympathetic and enteric nervous systems (Pachnis et al., Development, 1993, 119:1005).
Hirschsprung's disease, which is an apriority congenital megacolon, occurs by an RET dysfunction induced by germline mutation (Manie et al., Trends Genet. 2001, 17:580). Cancer such as multiple endocrine neoplasia (MEN) types 2A and 2B, and familial medullary thyroid carcinoma (FMTC) occurs by a hyper-function by RET mutation. Also, RET is considered as a molecular target to thyoid cancer (Cote and Gagel, N. Engl. J. Med., 2003, 349:1566).